Many dairy products, such as cottage cheese, sour cream, chip dip and the like are packaged in plastic containers having a complementary plastic lid or closure which engages with and seals the container. U.S. Pat. No. 4,826,039 and 4,872,586 disclose a container closure that is injection molded and designed to be used with the pre-existing conventional packaging equipment that had been used with automatic filling and capping equipment to place thermoformed closures onto the filled plastic containers. The dairies that use this equipment with thermoformed closures do want to replace their existing filling and capping equipment merely to switch from thermoformed closures to the injection molded closures. The thermoformed closures or lids are made by a die forcing a plastic sheet into the desired plug configuration, which has an enlarged sealing bead adjacent the bottom of the plug. The edges of the thermoformed sheet are later rolled to form the closure rim having a depending rim skirt with a generally circular cross-section. These thermoformed lids are nested in stacks from which they are dispensed. In the stack, the edges of the adjacent rim skirts are spaced from each other. That is, the outer skirts of the thermoformed containers do not extend downwardly as far as the plug portions. The nesting surfaces of adjacent thermoformed closures hold closures with this space between adjacent skirts; and it is into this space between adjacent skirts that a pair of mechanical fingers on a capping machine are inserted to remove the lowermost closure from the stack. With closures of this type, it is important that adjacent closures in the stack do not become askew or tilted with respect to one another, resulting in a relatively large gap between skirts on adjacent closures on one side of the stack and little or no gap between these adjacent closures on the other side of the stack. The mechanical fingers employed in automated packaging require generally uniform spacing between adjacent stacked closures for successful operation, and askew closures may cause obstruction of the mechanical fingers, resulting in production interruption and possibly equipment damage. U.S. Pat. No. 4,826,039 discloses a closure which, when stacked, maintains adjacent, stacked closures level with respect to one another, to provide generally uniform spacing between adjacent closures about their periphery.
In this patented closure, a central panel of the plug portion has a circular array of vertically extending ribs projecting upwardly from the central panel. If a closure begins to shift laterally relative to another closure in the stack, it will be supported against tilting by engaging supporting ribs on the closure therebelow. More specifically lower nesting surfaces on an offset in the upstanding sidewall of the plug-like portion will engage ribs on the rim of the closure therebelow. In this canted or inward position of the top lid, the ribs supported the upper lid from dropping further to the extent that caused its rim skirt to close or to substantially narrow the gap or space, into which the mechanical fingers need to move. If gap or space was allowed to become too narrow, the mechanical fingers would hit the inclined skirt wall and not remove the lid. When the upper lid shifted inwardly and was slightly canted enough to rest on the adjacent ribs of the lower lid, the space between adjacent skirts was still large enough to accept the mechanical fingers. Thus, the ribs functioned to prevent closure jamming.
It is preferred to support the closures in the stack by the continuous, circular nesting surface on the container rim rather than support a stack of closures on the small, top edges of the ribs. It will be appreciated that these closures are often shipped in large cartons and that cartons may be placed on top of each, and thereby cause a considerable downward force that would tend to force the top edges of the ribs into the panel of the overlying closure. Also, the closures should be capable of withstanding a drop test simulating the dropping of a carton full of stack closures, e.g. simulating a drop of the carton from a truck or the like for about four feet.
While the injection molded closures of the kind taught by U.S. Pat. No. 4,826,039 are commercially successful and in widespread use, the ribs on the top panel were annoying to people who picked up the closures two at a time with their hands and cut their fingers on the sharp, top edges of the ribs. It sometimes occurs in the dairy that a small short run will be capped manually by workers using their hands to place the lids onto the containers. In other instances, the capping machine may not have successfully capped the container with a closure, and a person will manually place a closure on that container to manually cap the closure. Persons who handled these closures wore leather gloves in some instances or put bandages on their fingertips to prevent their fingertips from being cut by the sharp, upper edges of the ribs. While the sharp edges of the ribs could be blunted to prevent cutting of fingers, the ribs still suffered the shortcoming of being potential dust collectors.
Furthermore, the ribs on the upper closure surface may detract from the aesthetic appearance of the closure and make it difficult to print onto the closure. The ribs on the upper closure surface also make it difficult to wipe spillage off of the closure surface.
It is desirable to provide a closure which can be formed by injection molding and having means for preventing both jamming of adjacent closures, with the closure also providing a smooth upper closure panel to facilitate wiping away of spillage on the closure and providing good aesthetic appearance to the consumer.